Photosite with pinned photodiode

Abstract

A photosite is formed in a semiconductor substrate and includes a photodiode confined in a direction orthogonal to the surface of the substrate. The photodiode includes a semiconductor zone for storing charge that is formed in an upper semiconductor region having a first conductivity type and includes a main well of a second conductivity type opposite the first conductivity type and laterally pinned in a first direction parallel to the surface of the substrate. The photodiode further includes an additional semiconductor zone including an additional well having the second conductivity type that is buried under and makes contact with the main well.

What is claimed is: 
     
       1. A photosite, comprising:
 a semiconductor substrate of a first conductivity type and having an upper surface; and 
 a photodiode confined in a direction orthogonal to the upper surface of the substrate, comprising:
 a first well in the substrate and having a second conductivity type opposite the first conductivity type; 
 a semiconductor zone configured to store charge comprising a second well of the first conductivity type that is formed in the first well and laterally pinned in a first direction parallel to the surface of the substrate by the first well; 
 a third well in the substrate and having the second conductivity type placed between the first well and a first conductivity type region in a lower part of the substrate; and 
 an additional semiconductor zone configured to store charge comprising a fourth well of the first conductivity type that is formed in the third well and laterally pinned in the first direction by the third well, the fourth well buried under and making contact with the second well. 
 
 
     
     
       2. The photosite according to  claim 1 , wherein the fourth well has a lower dopant concentration than a dopant concentration in the second well. 
     
     
       3. The photosite according to  claim 2 , wherein the fourth well has a laterally pinned width greater than a laterally pinned width of the second well in the first direction. 
     
     
       4. The photosite according to  claim 3 , wherein the fourth well has a laterally pinned width in a second direction less than a laterally pinned width of the second well in the second direction, wherein the second direction is orthogonal to the first direction. 
     
     
       5. The photosite according to  claim 1 , wherein the fourth well comprises a vertical sequence of a plurality of wells of the first conductivity type including a first additional well buried under and making contact with the second well and a second additional well buried under and making contact with the first additional well. 
     
     
       6. The photosite according to  claim 5 , wherein the third well comprises a plurality of additional well regions of the second conductivity type including a first additional well region laterally pinning the first additional well and a second additional well region laterally pinning the second additional well. 
     
     
       7. The photosite according to  claim 6 , wherein the first additional well has a lower dopant concentration than a dopant concentration in the second well and the second additional well has a lower dopant concentration than a dopant concentration in the first additional well. 
     
     
       8. The photosite according to  claim 7 , wherein the first additional well has a laterally pinned width greater than a laterally pinned width of the second well in the first direction, and the second additional well has a laterally pinned width greater than the laterally pinned width of the first additional well in the first direction. 
     
     
       9. The photosite according to  claim 8 , wherein the first additional well has a laterally pinned width in a second direction substantially equal to a laterally pinned width of the second well in the second direction, wherein the second direction is orthogonal to the first direction, and the second additional well has a laterally pinned width less than the laterally pinned width of the first additional well in the second direction. 
     
     
       10. The photosite according to  claim 5 , wherein the vertical sequence further comprises a third additional well buried under and making contact with the second additional well. 
     
     
       11. The photosite according to  claim 10 , wherein the third well comprises a plurality of additional well regions of the second conductivity type including a first additional well region laterally pinning the first additional well, a second additional well region laterally pinning the second additional well and a third additional well region laterally pinning the third additional well. 
     
     
       12. The photosite according to  claim 11 , wherein the first additional well has a lower dopant concentration than a dopant concentration in the second well, the second additional well has a lower dopant concentration than a dopant concentration in the first additional well and the third additional well has a lower dopant concentration than a dopant concentration in the second additional well. 
     
     
       13. The photosite according to  claim 12 , wherein the first additional well has a laterally pinned width greater than a laterally pinned width of the second well in the first direction, the second additional well has a laterally pinned width greater than the laterally pinned width of the first additional well in the first direction, and the third additional well has a laterally pinned width substantially equal to the laterally pinned width of the second additional well in the first direction. 
     
     
       14. The photosite according to  claim 13 , wherein the first additional well has a laterally pinned width in a second direction substantially equal to a laterally pinned width of the second well in the second direction, wherein the second direction is orthogonal to the first direction, the second additional well has a laterally pinned width less than the laterally pinned width of the first additional well in the second direction, and the third additional well has a laterally pinned width greater than the laterally pinned width of the second additional well in the second direction. 
     
     
       15. The photosite according to  claim 1 , wherein the fourth well comprises a vertical sequence of a plurality of additional wells lying between the second well and the first conductivity type region in a lower part of the substrate, each additional well of the vertical sequence only making contact with a preceding additional well and a following additional well. 
     
     
       16. The photosite according to  claim 15 , in which the additional well buried immediately under and making contact with the second well has a lower dopant concentration than a dopant concentration in the second well, and each other additional well in the vertical sequence has a lower dopant concentration than a dopant concentration in the additional well under which it is buried. 
     
     
       17. The photosite according to  claim 15 , wherein the additional wells of the vertical sequence are laterally pinned in the first direction. 
     
     
       18. The photosite according to  claim 17 , wherein each additional well laterally pinned in the first direction is centered, in the first direction, with the well under which it is buried. 
     
     
       19. The photosite according to  claim 15 , wherein the additional wells of the vertical sequence are is laterally pinned in a pinning direction corresponding to a second direction parallel to the surface of the substrate and orthogonal to the first direction. 
     
     
       20. The photosite according to  claim 19 , wherein each additional well laterally pinned in the second direction is centered, in the second direction, with the additional well under which it is buried. 
     
     
       21. The photosite according to  claim 15 , wherein the second well possesses a pinned width in a lateral pinning direction, wherein each additional well possesses a corresponding pinned width in the lateral pinning direction, the corresponding pinned width of the additional well buried immediately under and making contact with the second well being larger than the pinned width of the second well, and each other additional well having a pinned width in the lateral pinning direction that is larger than the pinned width of the additional well immediately under which it is buried. 
     
     
       22. The photosite according to  claim 1 , wherein the photodiode is an element of a matrix array of photosites of an imaging device formed in the semiconductor substrate.